1. Field of the Invention
The present invention relates to an actively-controlled sound absorption panel system using movement-controlled reflective plate and in particular to an actively-controlled sound absorption panel system using movement-controlled reflective plate capable of use as an acoustically lined duct for noise reduction of aircraft engines, gas turbines for power generation and emergency use, turbo machinery and blowers etc, for noise reduction in ducts of blowers or air-conditioners etc or tunnels, and for noise reduction of roads, factories, works or residences where there are severe variations of noise source character, namely sound level and spectrum.
2. Description of the Related Art
Problems of environmental availability regarding aircraft noise in particular engine noise represent serious problems that must be solved for operation of current aircrafts and for development and entry into service of new types of aircraft such as second-generation supersonic aircraft or high subsonic aircraft of ultra high bypass ratio turbofan engine. Therefore, considerable research and development aimed at aircraft noise reduction is continuing up to the present. As such noise reduction techniques, various types of acoustic panel are known in which acoustic absorption materials or acoustic panel structures are used as acoustic liner typically in the intake and exhaust ducts of engines; however, in recent years, active noise control techniques have attracted attention, in which noise is cancelled or reduced actively in response to changes of noise source character, namely noise level and spectrum.
In a currently proposed active noise control technique for aircraft engines, for example a plurality of microphones and acoustic transducers are arranged upstream and downstream of the fan rotor and fan outlet guide vane stage of the jet engine, and a control output signal is supplied from a fan reference signal and an error signal detected by the acoustic transducers and microphones, and the signal actuates an air control valve installed in fan stages, so as to provide acoustic cancellation of fan tone noise by delivering a high-pressure air into tone generation devices (ref. WO 96/12269).
However, these prior art of fan noise reduction techniques are unsatisfactory on account of the following problems.
(1) Regarding acoustic absorption materials or acoustic panel structures, although absorption spectrum has been somewhat widened and peak values have also been increased by use of various types of sound absorption material and special design of the sound absorbent structure, the reduced spectrum peak values are still insufficient. In addition, the absorptive peak frequency of the panels is practically fixed, and therefore the panels have the drawback that they are not able to cope with changes of noise source character, namely spectrum and peak frequency demanded for engine noise reduction (the B.P.F. tone frequency on landing and on take-off changes by 30 to 40%.)
Also (2) as a turbofan engine noise reduction technique, various types of acoustical panels are used as acoustic liners of engine ducts, but, due to restrictions on the honeycomb depth of these panels, they are practically incapable of sound absorption in respect of sounds of lower frequency.
Furthermore, (3) regarding the change of acoustical absorption character of the panel in response with engine noise source changes, although an acoustical panel can achieve a noise reduction amount of about 8 dB at a tuned fan rotational speed, at other rotational speeds the amount of noise reduction is reduced by about half, so that high-level noise reduction cannot be achieved, corresponding to engine operation changes.
Also, since the conventional active noise control techniques was restricted to one noise component reduction of the fan noise spectrum, even if all these were reduced, a noise reduction of only some 1 to 2 dB was obtained by the techniques in terms of the noise evaluation unit used in the aircraft noise certification (evaluation in terms of O.A.SPLdB (A), which is a unit based on the noise unit of the aircraft noise restriction value). Also, they have the drawback that an active control system aimed at reducing various acoustic modes of fan blade passing frequency tone and harmonics becomes complicated and expensive. Also, due to the limitation of acoustic output power level of the cancellation noise sound, intensive fan tones of 150 to 160 dB cannot be reduced by only active noise control system. A further problem is the failure of fan noise reduction due to occurrences of mis-tuning of the active cancellation of fan noise and of response lag of the active system. Furthermore, in the case of conventional active sound absorption panel techniques, the absorptive spectrum peak level and the peak frequency shift of the active sound absorption panel were small, so that the conventional active sound absorption panel technique could not be adapted in response to changes of fan rotational speed. Also, there were problems such as that the sound absorption level of low frequency sound below 800 Hz was extremely small and the absorptive frequency range was narrow, and the absorptive spectral level was small.